An ion source is a critical component of an ion implanter. The ion source generates an ion beam which passes through the beamline of the ion implanter and is delivered to a semiconductor wafer. The ion source is required to generate a stable, well-defined beam for a variety of different ion species and extraction voltages. In a semiconductor production facility, the ion implanter, including the ion source, is required to operate for extended periods without the need for maintenance or repair.
Ion implanters have conventionally used ion sources with directly heated cathodes, wherein a filament for emitting electrons is mounted in the arc chamber of the ion source and is exposed to the highly corrosive plasma in the arc chamber. Such directly heated cathodes typically constitute a relatively small diameter wire filament and therefore degrade or fail in the corrosive environment of the arc chamber in a relatively short time. As a result, the lifetime of the directly heated cathode ion source is limited. As used herein, source “lifetime” refers to the time before repair or replacement of the ion source.
Indirectly heated cathode ion sources have been developed in order to improve ion source lifetimes in ion implanters. An indirectly heated cathode includes a relatively massive cathode which is heated by electron bombardment from a filament and emits electrons themionically. The filament is isolated from the plasma in the arc chamber and thus has a long lifetime. Although the cathode is exposed to the corrosive environment of the arc chamber, its relatively massive structure insures operation over an extended period.
The cathode in the indirectly heated cathode ion source must be electrically isolated from its surroundings, electrically connected to a power supply and thermally isolated from its surroundings to inhibit cooling which would cause it to stop emitting electrons. Known prior art indirectly heated cathode designs utilize a cathode in the form of a disk supported at its outer periphery by a thin wall tube of approximately the same diameter as the disk. The tube has a thin wall in order to reduce its cross-sectional area and thereby reduce the conduction of heat away from the hot cathode. The thin tube typically has cutouts along its length to act as insulating breaks and to reduce the conduction of heat away from the cathode.
The tube used to support the cathode does not emit electrons, but has a large surface area, much of it at high temperature. This area loses heat by radiation, which is the primary way that the cathode loses heat. The large diameter of the tube increases the size and complexity of the structure used to clamp and connect to the cathode. One known cathode support includes three parts and requires threads to assemble.
Another indirectly heated cathode configuration is disclosed in International Publication No. WO 01/88946 published Nov. 22, 2001. A disk-shaped cathode is supported by a single rod at or near its center. A cathode insulator electrically and thermally isolates the cathode from an arc chamber housing. The disclosed cathode assembly provides highly satisfactory operation under a variety of operating conditions. However, in certain applications, deposits of contaminants on the insulator may cause a short circuit between the cathode and the arc chamber housing, thereby requiring repair or replacement of the ion source.
All of the known prior art indirectly heated cathode ion sources have had one or more disadvantages, including, but not limited to, short operating lifetimes and excessive complexity. Accordingly, there is a need for improved indirectly heated cathode ion sources.